Quantifying magma overpressure beneath a submarine caldera: A mechanical modeling approach to tsunamigenic trapdoor faulting near Kita-Ioto Island, Japan
Quantifying magma overpressure beneath a submarine caldera: A mechanical modeling approach to tsunamigenic trapdoor faulting near Kita-Ioto Island, Japan
Osamu SANDANBATA1,2・Tatsuhiko SAITO 2
1. Earthquake Research Institute, The University of Tokyo,
2. National Research Institute for Earth Science and Disaster Resilience
Monitoring submarine volcanoes is essential to understand and prepare for potential volcanic hazards in/around oceans, but it’s challenging because these volcanoes are in inaccessible environments. In a submarine volcano, named Kita-Ioto Caldera, with a caldera structure in the south of Japan, unusual volcanic earthquakes took place every several years (Figure 1a–c). After one of these earthquakes in 2008, a pressure sensor deployed on the sea bottom recorded a clear signal of tsunami waves (Figure 1d). By utilizing the tsunami signal from the earthquake, we attempt to measure how much magma pressure was building up beneath the volcano before the earthquake. By assuming that the earthquake happened with a so-called trapdoor faulting mechanism, or sudden rupture on an intra-caldera fault system due to highly pressurized magma beneath the volcano, we developed a method to assess the built-up magma pressure through quantification of the earthquake and tsunami sizes (Figure 2a). By applying the method, we successfully reproduced the observed tsunami waveform record with the earthquake model (Figure 2b) and estimated that the volcanic edifice was under a highly stressed condition before the earthquake (Figure 2c), suggesting the active magma accumulation process that has continued beneath the volcano. Signals emitted from volcanic earthquakes under oceans shed light on the activity of poorly monitored submarine volcanoes.
Figure 1. (a) The Izu-Bonin Arc, a volcanic arc extending to the south of Japan, and the location of the tsunami gauge (DART 52404) used in this study. (b) A volcanic archipelago consisting of Kita-Ioto Island, Ioto-Island (Iwo Jima), and Minami-Ioto Island (Minami-Iwo Jima). (c) The bathymetry around Kita-Ioto Caldera and seismic activity in the vicinity, obtained from the Global CMT catalog. (d) Tsunami waveforms recorded by the tsunami gauge DART 52404. Clear oscillations caused by the tsunami are recorded after the estimated tsunami arrival time.Figure 2. (a) Earthquake model of trapdoor faulting in Kita-Ioto Caldera. This model illustrates the trapdoor faulting, consisting of intra-caldera fault slip and deformation of the magma reservoir. (b) Comparison of observed tsunami waveforms and simulated tsunami waveforms based on the earthquake model. The simulated waveform sufficiently explains the observation. (c) Predicted deformation beneath the caldera before and after trapdoor faulting (cross-sectional view along the southeast-northwest line). (Top) Before the occurrence of faulting, the magma reservoir expands due to the inner increased magma pressure, leading to significant deformation of the surrounding host rock. Shear stress concentrates along the caldera fault. However, at this point, the fault remains locked, and no fault slip has occurred. (Bottom) When trapdoor faulting occurs, the intra-caldera fault causes sudden slip, inducing the magma reservoir deformation. As a result, the interior of the caldera along the fault moves upward significantly by meters, lifting seawater and generating a tsunami. Simultaneously, seismic waves are radiated due to fault rupture, and an earthquake of magnitude M~5 was observed.
